1. Field of the Invention
The invention relates to microwave energy attenuating circuits and microwave measuring devices including an attenuating circuit and a process for making the same.
2. Discussion of Background
A microwave energy attenuating circuit is generally made by a waveguide inside of which one or more plungers consisting of an absorbing material are made to slide through openings made on a wall. The attenuation depends on the dimensions of the plunger inside the guide. The absorbing materials are generally either graphite or resins with a glass fiber base filled with graphite. Variable attenuators are obtained by coupling the plunger or plungers to a micrometer screw which makes it possible to regulate more or less the sinking of the plunger inside the guide.
The drawback of such attenuators resides in the fact that the waves attenuated by the plunger are also phase shifted by the plunger. Now, in numerous applications, the phase shift that this type of attenuator introduces is unacceptable.
There are also precision-type attenuators, i.e., attenuators that do not phase shift the waves. These attenuators also consist of a waveguide inside which a metallized mica sheet can turn around a stationary plate placed in a plane perpendicular to the electric field. The drawback of such attenuators comes from the fact that they cannot receive an energy power of more than one watt at 100 GHz or more than several watts in the lower bands. Beyond this limit the mica sheet is burned and the tube heats up.
On the other hand, it is known for klystron-type microwave tubes to vary the Q factor of a resonant cavity by increasing the energy losses in this cavity, i.e., by causing an attenuation inside the cavity. Actually, what occurs is an attenuation of the microwave energy inside the cavity in order to increase the passband of the cavity. For this purpose, a technique is used which consists in lining the interior of the cavity with adhesive strips on which are deposited iron filings, the iron being a good absorbent. Subsequently sintering is performed so that by firing, at the molecular level there is an interpenetration of the filings into the material constituting the cavity. This material is generally copper.
Because these cavities are under vacuum, this solution cannot be used for attenuating circuits which are not under the same conditions and which often, on the contrary, are in a humid environment. In such an environment oxidation, blistering and lastly a detachment of the deposit occurs.
Besides the drawbacks that have just been described, no existing attenuating circuit has all of the following characteristics:
constant attenuation independent of the frequency of the waves in a given band located around 100 GHz
capability of receiving more than 1 W in this range of frequencies;
capability of allowing an attenuation of at least 15 to 20 dB;
no alteration due to a humid atmosphere; and
no involuntary phase shift during attenuation.
This invention makes it possible to remedy these problems.